Coating method and apparatus

ABSTRACT

The coating apparatus and method enable the coating liquid having been scraped-off to be reused without doing any one of the fluid adjusting treatment and the filtering treatment, while maintaining the features of the scraping-off type of extrusion coater, which is suitable for obtaining a uniform and extremely thin coating film. The coating head is provided with two slits: a coating slit and a recovering slit, and the excessive coating liquid having been discharged through the coating slit and applied to the web is scraped-off and recovered through the recovering slit.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to coating method and apparatus, inparticular to coating method and apparatus for use in manufacturingphotosensitive materials and magnetic recording media.

[0003] 2. Description of the Related Art

[0004] Photosensitive materials and magnetic recording media aremanufactured through a coating step of forming a coating film byapplying a prescribed coating liquid, such as magnetic liquid, to acontinuously running sheet substrate strip (hereinafter referred to as“web”). In recent years, magnetic recording media such as magneticrecording tape particularly for broadcasting or computer use have beenrapidly improved in capacity and recording density, and there has beenneeded a coating technology capable of providing a magnetic layer whichis extremely thin and of which surface is smooth.

[0005] Coating apparatus for applying a coating liquid to the surface ofa web include, for example, apparatus of types such as roll coater,gravure coating, roll coating plus doctor roll, extrusion coating, andslide coating; in recent years, apparatus of extrusion type have beencommonly used.

[0006] One method which employs an extrusion type of apparatus and inwhich the tip of a coating head is pushed against a web, as described inJapanese Patent Application Publication No. 58-109162, has been commonlyused in the field of manufacturing magnetic recording media, since athin and uniform coating layer can be obtained just by increasing thefluid pressure at the tip of the coating head utilizing the web tension,thereby eliminating the air accompanying the web. However, even in themethod of applying a single coat of coating liquid using this type ofcoating head, the reduction in coating thickness is limited.

[0007] As a further method for forming a much thinner coating layer,there is a method in which a very thin coating layer can be formed byapplying a coating liquid to a web to excess using a web-pushing type ofextrusion coater and then scraping-off the excess coating liquid with ablade disposed on the downstream side of the extrusion coater, asdescribed in Japanese Patent Application Publication No. 7-287843. Inthis case, the device which applies a coating liquid to a web to excessis not necessarily limited to the web-pushing type of extrusion coater,but coaters such as roller coater type coater, gravure coater andextrusion coater equipped with a back-up roll can be used.

[0008] In the method using the conventional scraping-off type ofextrusion coater shown in FIG. 8, however, since the coating liquidrecovered by scraping-off, with a blade 6, the excess amount of coatingliquid having been applied to a web 5 at a coating head 3 has been onceexposed to the atmosphere, the solvent contained in the coating liquidhas been volatilized, thereby changes are caused in the fluid physicalproperties such as viscosity and solid content concentration. Thus, themethod has a disadvantage in that, since the use of the coating liquid,as it is, of which viscosity has been changed adversely affects theamount of the coating liquid to be scraped-off or the final thickness ofthe coating film after being scraped-off with the blade 6, the recoveredcoating liquid must undergo treatment for adjusting its physicalproperties such as viscosity and solid content concentration in acoating liquid adjusting tank 8 before reused. In addition, since therecovering line for recovering the coating liquid is opened to theatmosphere, dust from the outside of the recovering line is likely to beincluded in the recovered coating liquid. Thus, the method also has adisadvantage in that dust cannot be fully removed even if the recoveredcoating liquid is filtered through a filter 9 when pumping the recoveredcoating liquid from a supply tank 1 to the coating head 3 with a pump 2.

SUMMARY OF THE INVENTION

[0009] The present invention has been made in the light of suchcircumstances, and an object of the present invention is to providecoating method and apparatus which enable the reuse of the coatingliquid having been scraped-off from a web without doing any one of thecoating liquid adjusting treatment and the filtering treatment, whilemaintaining the features of the scraping-off type of extrusion coater,which is suitable for obtaining a uniform and extremely thin coatingfilm layer.

[0010] Another object of the invention is to provide a coating apparatuswhich enables the obtaining of a uniform and extremely thin coating filmlayer equal to or superior to that provided by the scraping-off typeextrusion coater equipped with a doctor blade and enables the reuse ofthe coating liquid having been recovered from a web without doing anyone of the coating liquid adjusting treatment and the filteringtreatment.

[0011] In order to accomplish the above-described object, the presentinvention is directed to a coating method, comprising the steps of:pre-coating one side of a continuously running sheet substrate stripwith a coating liquid, using a pre-coating device, in an amount inexcess of that capable of obtaining an intended final coating thickness;and then scraping-off an excess amount of coating liquid with ascraping-off device provided on a downstream side along a runningdirection of the substrate relative to the pre-coating device, whereinthe coating liquid having been applied to the substrate is not opened tothe atmosphere during a time from the pre-coating until thescraping-off.

[0012] According to the present invention, the coating liquid havingbeen scraped-off from the continuously running substrate strip can bereused without doing any one of the coating liquid adjusting treatmentand the filtering treatment, while maintaining the features of thescraping-off type of extrusion coater, which is suitable for obtaining auniform and extremely thin coating film layer, since the coating liquidhaving been applied to the substrate is not opened to the atmosphereduring the time from the pre-coating of the substrate with the coatingliquid until the scraping-off of the express amount of the coatingliquid.

[0013] The term “the coating liquid having been applied to the substrateis not opened to the atmosphere” herein used means the state in which atleast the surface of the substrate to which the coating liquid has beenapplied is not opened to the atmosphere, and it does not matter whetherthe lateral edge portions of the substrate are opened to the atmosphereor not. And this is the same in the following.

[0014] In an aspect of the present invention, the coating liquid havingbeen scraped-off is recovered through a recovering line as a closedsystem and the recovered coating liquid is reused. Allowing therecovering line to be a closed system enables the fluid physicalproperties, such as viscosity and solid content concentration, of thecoating liquid to be reused to be much less changed and dust and thelike to be not included in the coating liquid.

[0015] In an aspect of the present invention, a desired coatingthickness is obtained by adjusting a fluid resistance of the recoveringline. Changing the fluid resistance of the recovering line enables thecoating thickness to be varied. This case also embraces one form of themethod in which the adjustment of the fluid resistance of the recoveringline is combined with at least any one of the adjustment of the coatingspeed, the adjustment of the coating liquid viscosity and the adjustmentof the solid content concentration.

[0016] In an aspect of the present invention, a desired coatingthickness is obtained by adjusting at least one of the amount of thecoating liquid to be used for the pre-coating and an amount of coatingliquid to be scraped-off. This method enables a desired coatingthickness to be obtained in a simple and easy manner.

[0017] In an aspect of the present invention, the amount of the coatingliquid used for the pre-coating and the amount of the coating liquidscraped-off are measured with flowmeters, respectively, and at least oneof the amounts is adjusted to allow a difference between the twomeasured amounts to provide the desired coating thickness. This methodenables desired coating thickness to be obtained with a high accuracy.

[0018] In an aspect of the present invention, the desired coatingthickness is obtained by adjusting a running speed of the substrate.Adjusting the running speed of the substrate (i.e., a coating speed)enables the coating thickness to be varied.

[0019] In an aspect of the present invention, the desired coatingthickness is obtained by adjusting a viscosity of the coating liquid.Adjusting the viscosity of the coating liquid enables the coatingthickness to be varied. This case also embraces one aspect of the methodin which the adjustment of the coating liquid viscosity is combined withthe adjustment of the coating speed.

[0020] In an aspect of the present invention, the desired coatingthickness is obtained by adjusting a solid content concentration of thecoating liquid. Changing the solid content concentration of the coatingliquid enables the coating thickness to be varied. This case alsoembraces one aspect of the method in which the adjustment of the solidcontent concentration is combined with at least either one of theadjustment of the coating speed and the adjustment of the coating liquidviscosity.

[0021] In an aspect of the present invention, the scraping-off device isconstructed so that the scraping-off device scrapes-off the excessamount of coating liquid by allowing a scraping-off portion thereof andthe substrate to be pushed relative to each other, thereby pressing thecoating liquid which has been applied to the substrate, the desiredcoating thickness being obtained by varying a pressing force applied tothe coating liquid. This case also embraces one aspect of the method inwhich the adjustment of the pressing force applied to the coating liquidis combined with at least any one of the adjustment of the fluidresistance of the recovering line, the adjustment of the solid contentconcentration of the coating liquid, the adjustment of the coatingliquid viscosity, the adjustment of at least either one of the amount ofthe coating liquid used for pre-coating and the amount of the coatingliquid scraped-off and the adjustment of the running speed of the sheetsubstrate strip.

[0022] In an aspect of the present invention, the pressing force isobtained by lapping the substrate over the scraping-off portion andvaried by adjusting a tensile force of the substrate exerted in therunning direction of the substrate.

[0023] In an aspect of the present invention, the pressing force isobtained by arranging a back-up roller on the back side of thesubstrate, to which no coating liquid is applied, in such a manner as toface the scraping-off portion and is adjusted by adjusting the clearancebetween the scraping-off portion and the back-up roller.

[0024] In an aspect of the present invention, the pressing force isobtained by arranging a back-up roller made of an elastic material onthe back side of the substrate, to which no coating liquid is applied,in such a manner as to face the scraping-off portion and is adjusted byadjusting at least one of a longitudinal elasticity modulus and ahardness of the back-up roller.

[0025] In an aspect of the present invention, the pressing force isobtained by arranging a spraying device which sprays fluid on the backside of the substrate, to which no coating liquid is applied, in such amanner as to face the scraping-off portion and is adjusted by adjustinga spraying pressure of the spraying device.

[0026] In an aspect of the present invention, the coating liquid is amagnetic coating liquid with magnetic particles dispersed therein. Thecoating method of the present invention is extremely effective incoating the substrate with a magnetic coating liquid which requiresaccuracy in the adjustment of the physical properties such as viscosityand solid content concentration.

[0027] In order to accomplish the above-described object, the presentinvention is also directed to a coating apparatus, comprising: apre-coating device which pre-coats one side of a continuously runningsheet substrate strip with a coating liquid in an amount in excess ofthat capable of obtaining an intended final coating thickness; ascraping-off device which scrapes-off an excess amount of coatingliquid, the scraping-off device being arranged on a downstream sidealong a running direction of the substrate relative to the pre-coatingdevice; and a closing mechanism which prevents the coating liquid havingbeen applied to the substrate from being opened to the atmospherebetween the pre-coating device and the scraping-off device.

[0028] According to the present invention, the coating liquid havingbeen scraped-off from the continuously running substrate strip can bereused without doing any one of the coating liquid adjusting treatmentand the filtering treatment, while maintaining the features of thescraping-off type of extrusion coater, which is suitable for obtaining auniform and extremely thin coating film layer, since a mechanism isprovided for preventing the coating liquid having been applied to thesubstrate from being opened to the atmosphere during the time from thepre-coating of the substrate with the coating liquid until thescraping-off of the excess amount of the coating liquid.

[0029] In an aspect of the present invention, a pre-coating portion ofthe pre-coating device and a scraping-off portion of the scraping-offdevice are integrated in a coating head; the closing mechanism is formedby allowing a lip surface of the coating head and the substrate to beadjacent to each other; and the pre-coating portion is arranged on theupstream side of the coating head, and the scraping-off portion isarranged on the downstream side of the coating head. The closingmechanism thus formed and constructed enables the coating liquid havingbeen applied to the substrate not to be opened to the atmosphere duringthe time from the pre-coating of the substrate with the coating liquiduntil the scraping-off of the express amount of the coating liquid,since the coating liquid applied to the substrate to excess at thepre-coating portion is moved to the scraping-off portion, where theexcess amount of the same is scraped-off, in such a state that it issandwiched between the substrate and the lip surface.

[0030] In an aspect of the present invention, the pre-coating portion isprovided with a coating slit for discharging the coating liquid; and thescraping-off portion is provided with a recovering slit for recoveringthe coating liquid having been scraped-off. Thus, all the paths of thepre-coating device and the scraping-off device can be a closed system.

[0031] In an aspect of the present invention, the coating apparatusfurther comprises: a pair of guide rollers which laps the substrate overthe lip surface of the coating head, the pair of guide rollers beingarranged on upstream and downstream sides relative to the coating head,wherein the desired thickness is obtained by adjusting a tensile forceof the substrate exerted in the running direction of the substrate topress the coating liquid having been applied to the substrate.

[0032] In an aspect of the present invention, the coating apparatusfurther comprises: a back-up roller which is arranged on a back side ofthe substrate, to which no coating liquid is applied, in such a manneras to face the lip surface of the coating head, wherein the desiredthickness is obtained by adjusting a clearance between the lip surfaceand the back-up roller to press the coating liquid having been appliedto the substrate.

[0033] In an aspect of the present invention, the coating slit is incommunication with a supplying line for supplying the coating liquid tobe used for pre-coating; the recovering slit is in communication with arecovering line for recovering the coating liquid having beenscraped-off; the supplying line and the recovering line are providedwith flowmeters to measure the amount of the coating liquid used forpre-coating and the amount of the coating liquid having beenscraped-off, respectively; and a flow control device is provided whichcontrols at least one of a flow of the supplying line and a flow of therecovering line to allow a difference between the measured flows toprovide the desired coating thickness. This coating apparatus enablesthe desired amount of the coating liquid to be applied to the substratewith a high accuracy.

[0034] In an aspect of the present invention, at least one of a doctorlip surface between the coating slit and the recovering slit and arecovering lip surface for scraping-off the excess amount of coatingliquid, both constituting the lip surface of the coating head, has ashape of one of a circular arc projecting toward the substrate, acombined surface of at least two planes successively provided andforming a vertex projecting toward the substrate, and a combined surfaceof a circular arc curvature projecting toward the substrate and a plane.This enables the fluid pressure at the lip surface of the coating headto be high. Thus, the air accompanying the substrate is eliminated, andthereby a thin and uniform coating layer can be obtained.

[0035] In an aspect of the present invention, a relation R₂>R₃ holds,where R₂ represents a radius of curvature of the doctor lip surface andR₃ represents a radius of curvature of the recovering lip surface. Thisenables the fluid pressure generated at the recovering lip surface to behigh, thereby enabling the coating liquid to be easily scraped-off.

[0036] In an aspect of the present invention, the scraping-off portionis provided with a rotating rod which comes in contact with the surfaceof the substrate to which the coating liquid has been applied andscrapes-off the excess amount of coating liquid. Thus, when foreignmatter is trapped between the rotating rod and the substrate, theforeign matter is easily eliminated, which results in reduction ofstreak defect.

[0037] In an aspect of the present invention, a pre-coating portion ofthe pre-coating device and a scraping-off portion of the scraping-offdevice are separately formed; and the closing mechanism comprises acovering member which covers at least the surface of the substrate towhich the coating liquid has been applied, the covering member beingarranged along a running route of the substrate between the pre-coatingportion and the scraping-off portion. This enables the coating liquidhaving been applied to the substrate not to be opened to the atmosphereduring the time from the completion of the pre-coating of the substratewith the coating liquid until the scraping-off of the express amount ofthe coating liquid, since the coating liquid applied to the substrate atthe pre-coating portion is moved to the scraping-off portion, where theexcess amount of the same is scraped-off, in such a state that it issandwiched between the substrate and the covering member. Accordingly,it can be constructed in such a manner as to enable the coating liquidhaving been applied to the substrate not to be opened to the atmosphereeven when the pre-coating portion of the pre-coating device and thescraping-off portion of the scraping-off device are formed separately.

[0038] In an aspect of the present invention, the pre-coating portion isprovided with a coating slit for discharging the coating liquid; thescraping-off portion is provided with a recovering slit for recoveringthe coating liquid having been scraped-off; and a back-up roller isprovided in such a manner as to face both of the pre-coating portion andthe scraping-off portion. This enables all the paths of the pre-coatingdevice and the scraping-off device to be a closed system even when thepre-coating portion and the scraping-off portion are separately formed,since providing the back-up roller and the guide member allows thepre-coating portion and the scraping-off portion to be constructed as ifthey were integrally formed.

[0039] In order to accomplish the above-described object, the presentinvention is also directed to a coating method, comprising the steps of:pre-coating one side of a continuously running sheet substrate stripwith a coating liquid, using a pre-coating device, in an amount inexcess of that capable of obtaining an intended final coating thickness;and then sucking-up an excess amount of coating liquid using a suctiondevice provided on a downstream side along a running direction of thesubstrate relative to the pre-coating device, wherein the coating liquidhaving been applied to the substrate is not opened to the atmosphereduring a time from the pre-coating until the sucking-up.

[0040] According to the present invention, one side of the continuouslyrunning sheet substrate strip is pre-coated with the coating liquid,using the pre-coating device, in the amount in excess of that capable ofobtaining an intended final coating thickness and then the excess amountof the coating liquid is sucked-up using the suction device provided onthe downstream side along the running direction of the substraterelative to the pre-coating device. As described so far, since thecoating method of the present invention forcibly sucks-up the excessamount of the coating liquid using the suction device, the thickness ofthe coating liquid applied to the substrate can be controlled to adesired coating thickness with a higher accuracy compared with theconventional scraping-off type of method which employs a doctor blade.Furthermore, the coating liquid having been sucked-up from thecontinuously running sheet substrate strip can be reused without doingany one of the coating liquid adjusting treatment and the filteringtreatment, since the coating liquid having been applied to the substrateis not opened to the atmosphere during the time from the completion ofthe pre-coating of the substrate with the coating liquid until thesucking-up of the excess amount of the coating liquid. Thus, a uniformand extremely thin coating film layer can be obtained which is equal toor superior to that obtained by the scraping-off type of method and therecovered coating liquid can be reused without doing any one of thecoating liquid adjusting treatment and the filtering treatment. In thiscase, in order to obtain the above-described desired coating thickness,preferably at least any one of the running speed of the substrate, andthe viscosity and the solid content concentration of the coating liquidis adjusted. Preferably the coating liquid is a magnetic coating withmagnetic particles dispersed therein.

[0041] In an aspect of the present invention, a sucking force of thesuction device is generated with a pump. In this method, the suckingforce can be controlled easily by adjusting the driving speed of thepump.

[0042] In an aspect of the present invention, the coating liquid havingbeen sucked-up with the suction device is recovered through a recoveringline as a closed system and the recovered coating liquid is reused.Forming the recovering line as a closed system enables the fluidphysical properties, such as viscosity and solid content concentration,of the coating liquid reused to be much less changed and makes it hardfor dust and the like to be included into the coating liquid.

[0043] In an aspect of the present invention, a wet thickness of thecoating liquid applied to the substrate through the pre-coating deviceis 3 mL/m² or more. On the substrate from which the excess amount of thecoating liquid has been sucked-up, a satisfactory coating film surface,which is free from streak defect, is likely to be formed.

[0044] In an aspect of the present invention, the desired coatingthickness is obtained by adjusting at least one of the amount of thecoating liquid to be used for pre-coating and an amount of the coatingliquid to be sucked-up. This coating method enables the desired coatingthickness to be obtained with a high accuracy.

[0045] In an aspect of the present invention, the amount of the coatingliquid used for pre-coating and the amount of the coating liquidsucked-up are measured with flowmeters, respectively, and at least oneof the amounts is adjusted to allow a difference between the twomeasured amounts to provide the desired coating thickness. This coatingmethod enables the desired coating thickness to be obtained with a highaccuracy. In this case, the adjustment is preferably conducted using apair of pumps: a suction pump as a suction device; and a supply pump forsupplying the coating liquid.

[0046] In order to accomplish the above-described object, the presentinvention is also directed to a coating apparatus, comprising: apre-coating device which pre-coats one side of a continuously runningsheet substrate strip with a coating liquid in an amount in excess ofthat capable of obtaining an intended final coating thickness; a suctiondevice which sucks-up an excess amount of coating liquid, the suctiondevice being arranged on a downstream side along a running direction ofthe substrate relative to the pre-coating device; and a closingmechanism which prevents the coating liquid having been applied to thesubstrate from being opened to the atmosphere between the pre-coatingdevice and the suction device.

[0047] According to the present invention, one side of the continuouslyrunning sheet substrate strip is pre-coated with the coating liquid,using the pre-coating device, in the amount in excess of that capable ofobtaining an intended final coating thickness and then the excess amountof the coating liquid is sucked-up using the suction device provided onthe downstream side along the running direction of the substraterelative to the pre-coating device. As described so far, since thecoating apparatus of the present invention forcibly sucks-up the excessamount of the coating liquid using the suction device, the amount of thecoating liquid applied to the substrate can be controlled to a desiredcoating amount with a higher accuracy compared with the conventionalscraping-off type of method which employs a doctor blade. Furthermore,the coating liquid having been sucked-up from the continuously runningsheet substrate strip can be reused without doing any one of the coatingliquid adjusting treatment and the filtering treatment, since theclosing mechanism is provided for preventing the coating liquid havingbeen applied to the substrate from being opened to the atmosphere duringthe time from the completion of the pre-coating of the substrate withthe coating liquid until the sucking-up of the excess amount of thecoating liquid.

[0048] In an aspect of the present invention, the sucking force of thesuction device is generated with a metering pump. The sucking force canbe controlled easily by adjusting the driving speed of the pump, andmoreover, the use of the metering pump enables the control of thesucking force to be more accurate.

[0049] In an aspect of the present invention, a pre-coating portion ofthe pre-coating device and a suction portion of the suction device areintegrated in a coating head; the closing mechanism is formed byallowing a lip surface of the coating head and the substrate to beadjacent to each other; and the pre-coating portion is arranged on theupstream side of the coating head, and the suction portion is arrangedon the downstream side of the coating head. This enables the coatingliquid having been applied to the substrate not to be opened to theatmosphere during the time from the completion of the pre-coating of thesubstrate with the coating liquid until the sucking-up of the excessamount of the coating liquid, since the coating liquid applied to thesubstrate at the pre-coating portion is moved to the suction portion,where the excess amount of the same is scraped-off, in such a state thatit is sandwiched between the substrate and the lip surface.

[0050] In an aspect of the present invention, the pre-coating portion isprovided with a coating slit for discharging the coating liquid; and thesuction portion is provided with a recovering slit for recovering thecoating liquid having been sucked-up. Thus, all the paths of thepre-coating device and the suction device can be a closed system.

[0051] In an aspect of the present invention, the coating apparatusfurther comprises a pair of guide rollers which laps the substrate overthe lip surface of the coating head, the pair of guide rollers beingarranged on upstream and downstream sides relative to the coating head.

[0052] In an aspect of the present invention, the coating apparatusfurther comprises a back-up roller which makes the substrate adjacent tothe lip surface of the coating head, the back-up roller being arrangedon a back side of the substrate, to which no coating liquid is applied.

[0053] In an aspect of the present invention, the coating slit is incommunication with a supplying line for supplying the coating liquid tobe used for pre-coating; the recovering slit is in communication with arecovering line for recovering the coating liquid having been sucked-up;the supplying line and the recovering line are provided with flowmetersto measure the amount of the coating liquid used for pre-coating and theamount of the coating liquid having been sucked-up, respectively; and aflow control device is provided which controls at least one of a flow ofthe supplying line and a flow of the recovering line to allow adifference between the measured flows to provide the desired coatingthickness. This coating apparatus enables the desired amount of thecoating liquid to be applied to the substrate with a high accuracy.

[0054] In an aspect of the present invention, a pre-coating portion ofthe pre-coating device and a suction portion of the suction device areseparately formed; and the closing mechanism comprises a covering memberwhich covers at least the surface of the substrate to which the coatingliquid has been applied, the covering member being arranged along arunning route of the substrate between the pre-coating portion and thesuction portion. Thus, since the coating liquid having been applied tothe substrate at the pre-coating portion is moved to the suctionportion, where the excess amount of coating liquid is sucked-up, in sucha state that it is sandwiched between the substrate and the coveringmember, the coating liquid having been applied to the substrate is notopened to the atmosphere between the pre-coating portion and the suctionportion. Accordingly, this coating apparatus enables the coating liquidhaving been applied to the substrate not to be opened to the atmosphereeven when the pre-coating portion and the suction portion are separatelyformed.

[0055] In an aspect of the present invention, the pre-coating portion isprovided with a coating slit for discharging the coating liquid; thesuction portion is provided with a recovering slit for recovering thecoating liquid having been sucked-off; and a back-up roller is providedin such a manner as to face both of the pre-coating portion and thesuction portion. This enables all the paths of the pre-coating deviceand the suction device to be a closed system even when the pre-coatingportion and the suction portion are separately formed, since the back-uproller and the guide member allows the pre-coating portion and thesuction portion to be constructed as if they were integrally formed.

[0056] The substrate used in the present invention may include the oneon which a primer has been applied and set in advance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The nature of the present invention, as well as other objects andadvantages thereof, will be explained in the following with reference tothe accompanying drawings, in which like reference characters designatethe same or similar parts throughout the figures and wherein:

[0058]FIG. 1 is a schematic view illustrating a first embodiment of thecoating apparatus according to the present invention, in which thepre-coating portion of a pre-coating device and the scraping-off portionof a scraping-off device are integrated into a coating head;

[0059]FIG. 2 is a partially cut-away perspective view illustrating thestructure of the coating head of FIG. 1 or 7;

[0060]FIG. 3 is a cross sectional view illustrating the lip surface ofthe coating head of FIG. 1 or 7;

[0061]FIG. 4 is a schematic view illustrating another aspect of theintegral coating apparatus of FIG. 1 or 7;

[0062]FIG. 5 is a schematic view illustrating another aspect of thescraping-off portion at which an excess coating liquid is scraped-offwith a rotating rod;

[0063]FIG. 6 is a schematic view illustrating another aspect of thecoating apparatus in the first or second embodiment of the presentinvention, in which a pre-coating portion and a scraping-off portion, ora pre-coating portion and a suction portion are separately formed;

[0064]FIG. 7 is a schematic view illustrating the second embodiment ofthe coating apparatus of the present invention, in which a pre-coatingportion of a pre-coating device and a suction portion of a suctiondevice are integrated into a coating head; and

[0065]FIG. 8 is a block diagram illustrating the entire construction ofthe conventional scraping-off type of coating apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0066] In the following, preferred embodiments of the present inventionwill be described in detail with reference to the accompanying drawings.

[0067]FIG. 1 is a schematic view illustrating the entire construction ofa first embodiment of the coating apparatus according to the presentinvention, in which a pre-coating portion 39 of a pre-coating device 31and a scraping-off portion 41 of a scraping-off device 33 are integratedinto a coating head 16. If the integrated coating head 16 is divided forexplanation into the pre-coating portion 39 and the scraping-off portion41, the two portions are marked off from each other with a broken line17 shown in the coating head 16.

[0068] As shown in FIG. 1, the coating apparatus 10 comprises a guideroller 14 for guiding a running web 12, a coating head 16, a pre-coatingdevice 31, a scraping-off device 33, and a controller 22 for controllingthe pre-coating device 31 and the scraping-off device 33. The coatinghead 16 is arranged in such a manner as to face the continuously runningweb 12 with its lip surface 24, which is provided on the tip thereof,adjacent to the web 12.

[0069] The pre-coating device 31 comprises a pre-coating portion 39integrally formed on the coating head 16 and a supplying line 18 forsupplying a coating liquid to the pre-coating portion 39 in an amount inexcess of the amount required for coating the web 12. And thescraping-off device 33 comprises a scraping-off portion 41 integrallyformed on the coating head 16 and a recovering line 20 for recoveringthe excess coating liquid having been scraped-off at the scraping-offportion 41.

[0070] As shown in FIGS. 1 and 2, within the coating head 16 a pair ofcylindrical pocket portions 26, 28, comprising a coating pocket portion26 and a recovering pocket portion 28, are formed in parallel with eachother in the widthwise direction of the web. Within the coating head 16,a coating slit 30 with a discharge port 30A in the lip surface 24 and arecovering slit 32 with a recovering port 32A in the lip surface 24 onthe downstream side along the running direction of the web 12 relativeto the discharge port 30A are also formed, and the coating slit 30 andthe recovering slit 32 are in communication with the coating pocket 26and the recovering pocket 28, respectively. Thus, the pre-coatingportion 39 and the scraping-off portion 41 are integrated into thecoating head 16. The coating pocket portion 26 is connected to thesupplying line 18 and the recovering pocket portion 28 is connected tothe recovering line 20. The coating slit 30 and the recovering slit 32are narrow passageways connecting the respective pocket portions 26, 28and the lip surface 24 and are extended in the widthwise direction ofthe web 12. A coating liquid is supplied from the supplying line 18 tothe coating pocket 26 of the coating head 16 in an amount in excess ofthe desired amount to be applied to the web 12, and the excess amount ofcoating liquid having been recovered through the recovering slit 32 intothe recovering pocket portion 28 is discharged into the recovering line20. While one type of coating head is illustrated in FIG. 2 in which, asa method for pumping a coating liquid into the coating pocket portion26, the coating liquid is supplied from one side of the coating pocketportion 26, there are other types in which a coating liquid is suppliedfrom one side of the coating pocket portion 26 and drawn from the otherside, or in which a coating liquid is supplied from the middle portionof the coating pocket portion 26 and branched into both sides. Any ofthese types of coating heads can be applied.

[0071] The supplying line 18 is such that its supplying piping 34connects a coating liquid tank 36 and the coating pocket portion 26 ofthe coating head 16 and in the middle of the supplying piping 34 areprovided a supply pump 38 for varying the amount of the coating liquidsupplied to the coating pocket portion 26 and a supply flowmeter 40 formeasuring the amount of the coating liquid flowing through the supplyingpiping 34, as shown in FIG. 1.

[0072] Meanwhile, the recovering line 20 is such that its recoveringpiping 42 connects the recovering pocket portion 28 and the coatingliquid tank 36 and in the middle of the recovering piping 42 areprovided a valve 44 for varying the amount of the coating liquidrecovered from the recovering pocket portion 28 by changing the flowresistance of the recovering line 20 and a recovery flowmeter 46 formeasuring the amount of the coating liquid flowing through therecovering piping 42. Thus, supplying/recovering lines 18, 20 as aclosed system which are not opened to the atmosphere are constructedbetween the coating head 16 and the coating liquid tank 36.

[0073] Guide rollers 14 are provided as a pair on both of the upstreamand downstream sides relative to the coating head 16 in the runningdirection of the web, respectively, and arranged in the positions lowerthan the tip of the coating head 16. Thus, the continuously running web12 is lapped over and made adjacent to the lip surface 24 of the coatinghead 16 in such a manner as to be pushed against the same. As a result,the coating liquid sandwiched between the web 12 and the lip surface 24is pressed. The pair of guide rollers 14, 14 are provided in such amanner as to be slidable in the slide direction 48, 48 shown in FIG. 1,enabling the tensile force of the web 12 exerted in the runningdirection of the web to be varied. As a sliding mechanism of the guiderollers 14, 14, any mechanisms can be employed as long as they canprovide the guide rollers with a slight sliding, and, for example, themechanism of a feed screw can be employed. Varying the tensile force ofthe web 12 exerted in the running direction of the web changes thepushing force of the web 12 against the lip surface 24 of the coatinghead 16, which in turn changes the pressing force applied to the coatingliquid sandwiched by the web 12 and the lip surface 24, resulting inchanges in the amount of the coating liquid scraped-off at thescraping-off portion 41. Thus, a desired coating thickness can beobtained. As described so far, the scraping-off device 33 in the presentinvention is constructed so that it scrapes-off the excess amount ofcoating liquid utilizing the edge portion of the recovering port 32A ofthe recovering slit 32 and a recovering lip surface 56 while pressingthe coating liquid having been applied to the web 12 with the lipsurface 24 of the coating head 16 pushed relative to the web 12, and thecoating liquid having been scraped-off is recovered through therecovering line 20. In this case, the shapes of a doctor lip surface 54between the coating slit 30 and the recovering slit 32 and of arecovering lip surface 56 on the downstream side relative to therecovering slit 32, seen along the running direction of the web, both ofwhich constitute the lip surface 24 of the coating head 16, preferablyhave curvatures in the form of a circular arc projecting toward the web12, as shown in FIG. 3. Although not shown in the figure, in addition toa circular arc curvature, each lip surface may have a combined surfaceof two or more planes successively provided and forming a vertexprojecting toward the web 12 or a combined surface of a circular arccurvature projecting toward the web 12 and a plane. Thus, the airaccompanying the web 12 is eliminated, and thereby a thin and uniformcoating layer can be obtained. Further, the relation, R₂>R₃ preferablyholds, where R₂ represents the radius of curvature of the doctor lipsurface 54 and R₃ the radius of curvature of the recovering lip surface56. This makes the pressure occurring on the recovering lip surface 56higher than that occurring on the doctor lip surface 54, therebyallowing the coating liquid to be scraped-off more easily. The blockshaving the doctor lip surface 54 and the recovering lip surface 56,respectively, are constructed using a rigid material such as carbide,fine ceramics, alumina A-150 or zirconia, or using members formed bycovering the blocks with any one of the materials, and the surfaces ofthe blocks are subjected to surface treatment so that they have asurface roughness, R_(max) of 0.5 μm or less, preferably 0.2 μm or less.

[0074] Into the controller 22 are inputted the measured values of theamount of the coating liquid supplied and the amount of the coatingliquid recovered, which are measured with a supply flowmeter 40 and arecovery flowmeter 46, respectively. Further, in the controller 22, anoperation is conducted to determine whether or not the difference inamount between the coating liquid having been supplied and the coatingliquid having been recovered corresponds to the desired amount of thecoating liquid applied to the web 12, and if the difference does notcorrespond to the desired amount, the controller 22 controls at leastone of the supply pump 38 and the valve 44 to make both amountcorrespond to each other.

[0075] In the following, the working of the first embodiment of thecoating apparatus 10 constructed as above will be described.

[0076] The coating liquid pumped out in excess from the coating liquidtank 36 with the supply pump 38, flowing through the supplying piping 34and supplied to the coating pocket portion 26 of the coating head 16rises up through the coating slit 30 and is discharged from thedischarge port 30A, while the amount of the coating liquid flowingthrough the supplying piping 34 is measured with the supply flowmeter 40and inputted into the controller 22. The coating liquid discharged toexcess from the discharge port 30A is applied to the web 12 whileforming a bead 50 between the lip surface 24 of the coating head 16 andthe web 12 running adjacent to the lip surface 24. In other words, thecoating liquid supplied to excess is applied to the web 12 in such astate that the discharge force which discharges the coating liquid fromthe discharge port 30A and pushing force of the web 12 against the lipsurface 24 of the coating head 16 are well balanced. Meanwhile, at therecovering port 32A of the recovering slit 32 formed in the lip surface24 on the downstream side relative to the discharge port 30A in therunning direction of the web, the excessive coating liquid isscraped-off into the recovering slit 32. Specifically, the coatingliquid applied to the web 12 to excess at the discharge port 30A of thecoating slit 30 accompanies the web 12 and reaches the recovering port32A of the recovering slit 32, where part of the coating liquid appliedto excess, that is, the excess coating liquid is scraped-off by the edgeportion of the recovering port 32A and the recovering lip surface 56,which will be described later. Thus, the coating liquid applied to theweb 12 to excess is divided into two: coating liquid flowing through therecovering slit 32; and coating liquid decreased to the desired amountand continuously accompanying the web 12. The coating liquid distributedinto the recovering slit 32 forms the stream of the coating liquid whichreaches the coating liquid tank 36 through the recovering pocket portion28 and the recovering line 20 and recovered into the coating liquid tank36. And the amount of the coating liquid flowing through the recoveringpiping 42 is measured with the recovery flowmeter 46 and inputted intothe controller 22.

[0077] In this coating/recovering operation, the controller 22 controlsat least one of the driving speed of the supply pump 38 and the amountof the valve 44 opening so as to allow the difference in amount, havingbeen measured with the supply flowmeter 40, between the coating liquidhaving been supplied and the coating liquid having been recovered andmeasured with the recovery flowmeter 46 to correspond to the desiredamount of the coating liquid to be applied to the web 12. Although notshown in FIG. 1, the controller 22 and the sliding mechanism of theguide rollers 14, 14 may be connected by radio or a signal cable toautomatically control the slide of the guide rollers 14, 14 with thecontroller 22. In this case, the sliding amounts of the guide rollersare not necessarily the same, and a desired thickness of the coatingliquid layer may be obtained by allowing the pushing force of the web 12against the recovering lip surface 56 to be larger than that against thedoctor lip surface 54 so as to apply much larger pressing force to thecoating liquid sandwiched between the web 12 and the recovering lipsurface 56, thereby varying the amount of the coating liquid to bescraped-off. Further, a desired thickness of the coating liquid layermay also be obtained by allowing the controller 22 to control all of thedriving speed of the supply pump 38, the amount of the valve 44 opening,and the sliding amount of the pair of guide rollers 14, 14 based on themeasured values with the supply flowmeter 40 and the recovery flowmeter46. Further, a desired thickness of the coating liquid layer may also beobtained by adjusting the running speed of the web 12, that is, thecoating speed. Still further, a desired thickness of the coating liquidlayer may also be obtained by adjusting the viscosity and the solidcontent concentration of the coating liquid. Further, a desiredthickness of the coating liquid layer may also be obtained by adjustingthe flow resistance of the recovering line 20. The flow resistance canbe adjusted by, for example, adjusting the amount of the opening of thevalve 44 provided on the recovering line 20. Further, a desiredthickness of the coating liquid layer may also be obtained by adjustingthe combination of coating speed, viscosity of the coating liquid, solidcontent concentration and flow resistance.

[0078] While in FIG. 1, the pressing force applied to the coating liquidsandwiched between the web 12 and the lip surface 24 of the coating head16 is adjusted by adjusting the tensile force of the web 12 exerted inthe running direction of the web with the pair of guide rollers 14, 14,FIG. 4 illustrates another aspect of applying pressuring force to thecoating liquid applied to the web 12. Specifically, a back-up roller 60is arranged on the backside of the web 12, to which no coating liquid isapplied, in such a manner as to face the coating head 16. The pressingforce is adjusted by sliding the back-up roller 60 back and forthrelative to the coating head 16 to vary the clearance between thecoating head 16 and the back-up roller 60. As a sliding mechanism of theback-up roller 60, any mechanisms can be employed as long as they canprovide the back-up roller with a slight sliding, and, for example, themechanism of a feed screw can be employed. In this case, the back-uproller may be formed out of an elastic material so as to adjust thepressing force by varying at least one of the modulus of longitudinalelasticity and the hardness of the back-up roller. Further, though notshown in the figure, a spraying device which sprays a fluid may bearranged on the back side of the web 12, to which no coating liquid isapplied, in such a manner as to face the coating head 16 to obtain apressing force and the pressing force may be varied by adjusting thespraying pressure of the spraying device.

[0079] While in FIG. 3 the lip surface is illustrated taking an examplewhere the shapes of the doctor lip surface 54 and the recovering lipsurface 56 of the lip surface 24 of the coating head 16 have curvaturesin the form of a circular arc projecting toward the web 12 so as toallow the coating liquid to be scraped-off easily, a rotating rod 62 maybe provided on the tip of the recovering lip surface 56 on therecovering slit 32 side, as shown in FIG. 5, to allow the coating liquidto be scraped-off easily. The rotating rod 62 can be rotated in the samedirection or in the reverse direction relative to the running directionof the web 12; however, preferably the rod is rotated in the reversedirection.

[0080] As described so far, in the first embodiment of the presentinvention, the coating head 16 is provided with two slits: the coatingslit 30 and the recovering slit 32, and the excess amount of coatingliquid having been applied to the web 12 by discharging the coatingliquid to excess from the coating slit 30 is scraped-off and recoveredthrough the recovering slit 32. Thus, since the coating liquid is beingsandwiched between the lip surface 24 of the coating head 16 and the web12 during the time from the completion of the pre-coating operationuntil the scraping-off operation, a closed system can be formed in whichthe coating liquid is not exposed to the atmosphere during the time fromthe completion of the pre-coating operation until the scraping-offoperation, which the prior art cannot accomplish. Further, as describedabove, the supplying/recovering lines 18, 20 as a closed system areconstructed between the coating head 16 and the coating liquid tank 36;accordingly, the entire coating apparatus 10 can be constructed as aclosed system. This makes it hard to expose the coating liquid havingbeen recovered to the atmosphere, which prevents the solvent in thecoating liquid from volatilizing, causing no changes in fluid physicalproperties such as viscosity and solid content concentration. Inaddition, dust and the like from the outside of the recovering linebecomes hard to be included in the coating liquid.

[0081] Furthermore, since the edge portion of the recovery port 32A ofthe recovering slit 32 and the recovering lip surface 56 on thedownstream side relative to the recovering slit 32 serve as ascraping-off surface of the blade of the coating apparatus currently inuse, the coating liquid having been scraped-off can be reused withoutdoing any one of the fluid adjusting treatment and the filteringtreatment, while maintaining the features of the scraping-off type ofextrusion coater, which is suitable for obtaining a uniform andextremely thin coating film. Accordingly, the coating apparatus 10 ofthe present invention is suitable to obtain a uniform and extremely thinmagnetic layer which is required in manufacturing magnetic recordingmedia and preferably the wet thickness of the magnetic coating liquidapplied to the web 12 is 5 μm or less. The coating apparatus 10 of thepresent invention is particularly suitable to apply a coating liquid tothe web 12 to thickness of, more preferably, as extremely thin as 2 μmor less.

[0082] Preferable coating conditions are as follows: the coating speedis 50 to 600 m/min, more preferably 50 to 300 m/min; the final thicknessof the magnetic coating liquid (dry thickness) is 0.5 μm or less, morepreferably 0.2 μm or less. And the fluid viscosity is 1 Pa.s (10 poise)or less, more preferably 0.2 Pa.s (2 poise) or less at 100 (1/sec); and0.01 Pa.s (10 centipoise) or less, more preferably 0.005 Pa.s (5centipoise) or less at 40000 (1/sec).

[0083]FIG. 6 is a schematic view illustrating another aspect of thefirst embodiment of the present invention, in which the pre-coatingportion 39 of a pre-coating device 31 and the scraping-off portion 41 ofa scraping-off device 33 are separately formed and arranged in such amanner as to be away from each other.

[0084] As shown in FIG. 6, according to a coating apparatus 65, the web12 runs with its back side born by a back-up roller 66 of a largediameter and the pre-coating portion 39 and the scraping-off portion 41are arranged away from each other and adjacent to the circumferentialsurface of the back-up roller 66. In the pre-coating portion 39 and thescraping-off portion 41, the coating pocket portion 26, the recoveringpocket portion 28, the coating slit 30 and the recovering slit 32 areformed similar to the coating apparatus 10 of FIG. 1. And an arc-shapedcover plate 68 is provided along the running route of the web, that is,along the circumferential surface of the back-up roller 66. The coatingapparatus 65 also includes the supplying line 18 and the recovering line20 as a closed system, which are not shown in FIG. 6 since they are thesame as those illustrated in FIG. 1.

[0085] According to the coating apparatus 65 constructed as above, thecoating liquid applied to the web 12 to excess through the coating slit30 of the pre-coating portion 39 moves to the scraping-off portion 41 instate where it is sandwiched between the web 12 and the cover plate 68,and the excess amount of the coating liquid is scraped-off into therecovering slit 32 at the scraping-off portion 41. Thus, in the coatingapparatus 65 of FIG. 6, similar to the coating apparatus 10 of FIG. 1,the coating liquid can avoid being exposed to the atmosphere during thetime from the completion of the coating operation to the scraping-offoperation. In this case, if the clearance between the web 12 and thecover plate 68 is too large, an air layer is formed between the coatingliquid and the cover plate 68, causing the solvent to be likely tovolatilize from the coating liquid. Accordingly, the clearance should bealmost as large as the coating thickness of the coating liquid appliedto the web 12 to excess. It is, however, essential that the clearance islarger than the intended final coating thickness.

[0086] While the coating apparatus has been described in this embodimenttaking an example of extrusion coater type of apparatus which isprovided with the coating slit 30 in its pre-coating portion 39, incases where the pre-coating portion 39 and the scraping-off portion 41are separately formed as shown in FIG. 6, coaters of various types suchas roll coater type, gravure coater type and bar coater can also beused. And for the scraping-off portion 41, a doctor blade, rod or thelike can be used. The most important thing is that the coating apparatusis constructed in such a manner as to allow the excess amount of coatingliquid applied to the web 12 not to be opened to the atmosphere until itis scraped-off. While in FIG. 6, the excess amount of the coating liquidis scraped-off with the edge portion of the recovering slit 32 of thescraping-off portion 41 and the recovering lip surface 56, the rotatingrod 62 as illustrated in FIG. 5 may also be provided on the scraping-offportion 41.

[0087] In the following, the coating apparatus of a second embodimentaccording to the present invention will be described. In the secondembodiment, the same parts as those of the first embodiment will bedescribed with like reference characters, and when the same drawings areused both in the first and second embodiments, the reference charactersof the second embodiment are put in parentheses.

[0088]FIG. 7 is a schematic view illustrating the entire construction ofthe coating apparatus according to the second embodiment of the presentinvention, in which the pre-coating portion 39 of the pre-coating device31 and a suction portion 141 of a suction device 133 are integrated intothe coating head 16. If the integrated coating head 16 is divided forexplanation into the pre-coating portion 39 and the suction portion 141,the two portions are marked off from each other with a broken line 17shown in the coating head 16.

[0089] As shown in FIG. 7, the coating apparatus 10 of the secondembodiment comprises the guide roller 14 for guiding the running web 12,the coating head 16, the pre-coating device 31, the suction device 133,and the controller 22 for controlling the pre-coating device 31 and thesuction device 133. The coating head 16 is arranged in such a manner asto face the continuously running web 12 with its lip surface 24, whichis provided on the tip thereof, adjacent to the web 12.

[0090] The pre-coating device 31 comprises the pre-coating portion 39formed on the coating head 16 and the supplying line 18 for supplyingthe coating liquid to the pre-coating portion 39 in an amount in excessof the amount required for coating the web 12. And the suction device133 comprises the suction portion 141 formed on the coating head 16 andthe recovering line 20 for recovering express amount of the coatingliquid sucked-up at the suction portion 141.

[0091] As shown in FIGS. 7 and 2, within the coating head 16 the pair ofcylindrical pocket portions 26, 28, comprising the coating pocketportion 26 and the recovering pocket portion 28, are formed in parallelwith each other in the widthwise direction of the web. Within thecoating head 16, the coating slit 30 with the discharge port 30A in thelip surface 24 and the recovering slit 32 with the recovering port 32Ain the lip surface 24 on the downstream side relative to the dischargeport 30A as seen along the running direction of the web 12 are alsoformed and the coating slit 30 and the recovering slit 32 are incommunication with the coating pocket 26 and the recovering pocket 28,respectively. Thus, the pre-coating portion 39 and the suction portion141 are integrated into the coating head 16. The coating pocket portion26 is connected to the supplying line 18 and the recovering pocketportion 28 is connected to the recovering line 20. The coating slit 30and the recovering slit 32 are narrow passageways connecting therespective pocket portions 26, 28 and the lip surface 24 and areextended in the widthwise direction of the web 12. A coating liquid issupplied from the supplying line 18 to the coating pocket 26 of thecoating head 16 in an amount in excess of the desired amount to beapplied to the web 12, and the excess amount of coating liquid havingbeen recovered through the recovering slit 32 into the recovering pocketportion 28 is discharged into the recovering line 20. Various aspects ofthe coating head 16 in which the coating liquid is supplied to thecoating pocket portion 26 are the same as those of the firs embodiment.

[0092] The description of the supplying line 18 is omitted since it isthe same as described in the first embodiment of FIG. 1. Meanwhile, therecovering line 20 is such that its recovering piping 42 connects therecovering pocket portion 28 and the coating liquid tank 36 and in themiddle of the recovering piping 42 are provided a suction pump 144 forvarying the amount of the coating liquid sucked-up from the recoveringslit 32 and the recovery flowmeter 46 for measuring the amount of thecoating liquid flowing through the recovering piping 42. Thus, thesupplying/recovering lines 18, 20 as a closed system which are notopened to the atmosphere are constructed between the coating head 16 andthe coating liquid tank 36.

[0093] The description of guide rollers 14 is omitted since they are thesame as described in the first embodiment of FIG. 1. While in FIG. 7 theweb 12 is allowed to be adjacent to the lip surface 24 of the coatinghead 16 by the pair of guide rollers 14, the back-up roller 60 may bearranged on the back side of the web 12, to which no coating liquid isapplied, in such a manner as to face the coating head 16. In this case,the back-up roller 60 may be formed out of an elastic material.

[0094] Into the controller 22 are inputted the measured values of theamount of the coating liquid supplied and the amount of the coatingliquid recovered, which are measured with the supply flowmeter 40 andthe recovery flowmeter 46, respectively. Further, in the controller 22,an operation is conducted to determine whether or not the difference inamount between the coating liquid having been supplied and the coatingliquid having been recovered corresponds to the desired amount of thecoating liquid applied to the web 12, and if the difference does notcorrespond to the desired amount, the controller 22 controls at leastone of the supply pump 38 and the suction pump 144 to make both amountcorrespond to each other.

[0095] In the following, the working of the second embodiment of thecoating apparatus 10 constructed as above will be described.

[0096] The coating liquid pumped out to excess from the coating liquidtank 36 with the supply pump 38 and supplied to the coating pocketportion 26 of the coating head 16 rises up through the coating slit 30and is discharged from the discharge port 30A, while the amount of thecoating liquid flowing through the supplying piping 34 is measured withthe supply flowmeter 40 and inputted into the controller 22. The coatingliquid discharged to excess from the discharge port 30A is applied tothe web 12 while forming a bead between the lip surface 24 of thecoating head 16 and the web 12 running adjacent to the lip surface 24.In other words, the coating liquid supplied to excess is applied to theweb 12 in such a state that the discharge force which discharges thecoating liquid from the discharge port 30A and pushing force of the web12 against the lip surface 24 of the coating head 16 are well balanced.Meanwhile, at the recovering port 32A of the recovering slit 32 formedin the lip surface 24 on the downstream side relative to the dischargeport 30A in the running direction of the web, the excess amount ofcoating liquid is sucked-up into the recovering slit 32 by the suckingforce of the suction pump 144.

[0097] The excess amount of the coating liquid sucked-up into therecovering slit 32 and accumulated in the recovering pocket portion 28is recovered into the coating liquid tank 36 through the recovering line20, while the amount of the coating liquid flowing through therecovering piping 42 is measured with the recovery flowmeter 46 andinputted into the controller 22.

[0098] In this coating/recovering operation, the controller 22 controlsat least one of the driving speed of the supply pump 38 and the drivingspeed of the suction pump 144 so as to allow the difference Q obtainedby subtracting the amount of the coating liquid recovered Q2, which ismeasured with the recovery flowmeter 46, from the amount of the coatingliquid supplied Q₁, which is measured with the supply flowmeter 40, tocorrespond to the desired amount of the coating liquid to be applied tothe web 12. Specifically, driving the supply pump 38 and the suctionpump 144 forms a circulating flow of the coating liquid starting at thecoating liquid tank 36→supplying piping 34→coating pocket portion26→coating slit 30→recovering slit 32→recovering pocket portion28→recovering piping 42 and reaching the coating liquid tank 36, andsince the amount of the coating liquid supplied Q₁ by the supply pump islarger than the amount of the coating liquid recovered Q₂ by the suctionpump 144, the flow of the coating liquid is divided into two: the flowinto the recovering slit 32 and the flow accompanying the web 12 at therecovery port 32A of the recovering slit 32. Accordingly, Q₁−Q₂=Q is thedesired amount of the coating liquid applied to the web 12, and auniform and extremely thin coating film layer can be obtained byallowing the value Q to be small. In this case, the wet thickness t₁ ofthe coating liquid, which has been excessively supplied, between the web12 and a doctor lip surface 54, which constitutes the lip surface 24together with a back lip surface 52 and a recovering lip surface 56 asshown in FIG. 3, or the wet thickness of the coating liquid before beingsucked into the recovering slit 32 is preferably 3 mL/m² or more(indicated with the amount of the coating liquid per 1 m²). The reasonis that if the wet thickness t₁ is too thin, fluid pressure (pressingforce applied to the coating liquid) sufficient to eliminate the airwhich accompanies the web 12 is not generated, as a result, defect suchas streak defect is likely to occur on the surface of the coating filmobtained on the web 12 as an end product.

[0099] The coating speed, the final thickness of the magnetic coatingliquid (dry thickness) and the coating liquid viscosity, as preferablecoating conditions, in this case are the same as described in the firstembodiment. The materials for and the surface roughness of the blocks onwhich the doctor lip surface 54 and the recovering lip surface 56 havebeen formed are also the same as described in the first embodiment.

[0100] As described so far, according to the second embodiment of thepresent invention, the coating head 16 is provided with two slits: thecoating slit 30 and the recovering slit 32, and the excess amount ofcoating liquid having been applied to the web 12 by discharging thecoating liquid to excess from the coating slit 30 is sucked-up andrecovered through the recovering slit 32. Thus, since the coating liquidis being sandwiched between the lip surface 24 of the coating head 16and the web 12 during the time from the completion of the pre-coatingoperation until the scraping-off operation, the line between the coatingslit 30, through which the coating liquid is applied to the web 12, andthe recovering slit 32, through which the excess amount of coatingliquid is sucked-up, can be a closed system where the coating liquid isnot exposed to the atmosphere. Further, as described above, thesupplying/recovering lines 18, 20, as a closed system, which are notopened to the atmosphere are constructed between the coating head 16 andthe coating liquid tank 36; accordingly, the entire coating apparatus 10can be a closed system line. This makes it hard for the solvent in thecoating liquid to volatilize, causing fewer changes in liquid physicalproperties such as viscosity and solid content concentration andpreventing dust and the like outside of the recovering line from beingincluded in the coating liquid. Furthermore, since the excess amount ofthe coating liquid is forcibly sucked-up into the recovering slit 32with the suction pump 144, the amount of the coating liquid sucked-up iseasy to control; thus the coating apparatus of the present invention issuitable to obtain a uniform and extremely thin coating film layer.

[0101] Thus, the coating apparatus of the present invention can providea uniform and extremely thin coating film layer which is equal orsuperior to that of the conventional scraping-off type of extrusioncoater using a doctor blade, and the recovered coating liquid can bereused without doing any one of the fluid adjusting treatment and thefiltering treatment. Accordingly, the coating apparatus 10 of thepresent invention is suitable to obtain a uniform and extremely thinmagnetic layer, which is required in manufacturing magnetic recordingmedia, and the wet thickness of the magnetic coating liquid applied tothe web 12 is preferably 5 μm or less. The coating apparatus 10 of thepresent invention is particularly suitable to apply a coating liquid tothe web 12 to the thickness of, more preferably, as extremely thin as 2μm or less.

[0102]FIG. 6 is a schematic view illustrating another aspect of thesecond embodiment of the present invention, in which the pre-coatingportion 39 and the suction portion 141 are separately formed andarranged in such a manner as to be away from each other. As seen fromFIG. 6, the coating apparatus 65 is constructed in the same manner asdescribed in the first embodiment, except that it has the suctionportion 141 instead of the scraping-off portion 41; accordingly, thedescription of the apparatus is omitted. According to the coatingapparatus 65 constructed as above, the coating liquid applied to the web12 to excess through the coating slit 30 of the pre-coating portion 39moves to the suction portion 141 in state where it is sandwiched betweenthe web 12 and the cover plate 68, and the excess amount of the coatingliquid is sucked into the recovering slit 32 at the suction portion 141.Thus, in the coating apparatus 65 of FIG. 6, similar to the coatingapparatus 10 of FIG. 7, the coating liquid can avoid being exposed tothe atmosphere during the time from the completion of the coatingoperation to the sucking operation. The clearance between the web 12 andthe cover plate 68 in this case is the same as described in the firstembodiment. In cases where the pre-coating portion 39 and thescraping-off portion 41 are separately formed as shown in FIG. 6,coaters of various types such as roll coater type, gravure coater typeand bar coater can also be used without using the extrusion coater typeas the pre-coating portion 39 as shown in FIG. 1.

EXAMPLES Example 1

[0103] Tests were conducted for obtaining a product with a thin coatingfilm layer using an integral coating apparatus shown in FIG. 1 in whichthe pre-coating portion and the scraping-off portion are integrated intothe coating head. And the coating thickness was also examined whenreusing as a coating liquid the coating liquid having been recovered atthe scraping-off portion without doing any one of the fluid adjustingtreatment and the filtering treatment.

[0104] (Coating Liquid Used for Tests)

[0105] Five types of coating liquid (A, B, C, D, E) shown in Table 2,which were different in viscosity and solid content concentration, wereprepared based on the formulation of coating liquid shown in Table 1 andused. TABLE 1 Formulation of Coating liquid Part by weight Cosubstituted BaFe 300 Average particle diameter 0.1 μm, Plates ratio 3.3,Average thickness 0.03 μm, Anti-magnetic force 660 Oe Polyvinylchloride-acetate - Maleic anhydride copolymer 45 (polymerization degree450) Amyl stearate 10 Lecithin 3 Chrome oxide 5 Methyl ethyl ketone(MEK) Adjusted depending on the viscosity and the Cyclohexanone solidcontent concentration of coating liquid

[0106] TABLE 2 Name of Viscosity Solid Content Coating liquid(40000/sec) Concentration (%) Main Solvent A 0.0081 Pa · s 33MEK:Cyclohexanone = 1:1 B 0.0055 Pa · s 26 MEK:Cyclohexanone = 1:1 C0.0039 Pa · s 23 MEK:Cyclohexanone = 1:1 D 0.0085 Pa · s 26Cyclohexanone E 0.0067 Pa · s 23 Cyclohexanone

[0107] (Testing Conditions)

[0108] A coating head with coating width of 1000 mm, coating slit gap of0.2 mm, recovering slit gap 0.5 mm and PET (polyethylene terephthalate)film 15 μm thick, as a web, were used.

[0109] Tests, test 1 to test 11, were conducted at 11 test areas underdifferent conditions with respect to 4 factors: type of coating liquid,coating speed, pressing force applied to coating liquid at scraping-offportion, and flow resistance of recovering line, and a comparison wasmade among the coating thickness values obtained by respective tests. Intest 1, the coating thickness was also examined when reusing therecovered coating liquid without doing any one of the coating liquidadjusting treatment and the filtering treatment.

[0110] The coating speed was varied within the range of 50 to 200 m/minand the flow resistance of the recovering line was varied whileadjusting the opening amount of the valve provided on the recoveringline. The pressing force against the coating liquid was varied whilevarying the tensile force exerted in the running direction of the webwithin the range of 7 to 30 kgf/m.

[0111] Test results are shown in Table 3. TABLE 3 Tensile Force ofAmount of Valve Coating Test Type of Coating Coating Speed Web OpeningThickness No. liquid m/min kgf/m (Full-open 100%) (μm) 1 A 100 24 1000.45 2 B 100 24 100 0.26 3 C 100 24 100 0.18 4 D 100 24 100 0.36 5 E 10024 100 0.31 6 C  50 24 100 0.12 7 C 200 24 100 0.41 8 C 100 17 100 0.249 C 100 30 100 0.14 10 C 100  7  60 0.26 11 C 100  7  80 0.55 12 Reuse100 24 100 0.45

[0112] As seen from the test results shown in Table 3, particularly fromthe comparison among the results of test 1 to 5 which were conductedunder the same conditions except only one factor, type of coatingliquid, the coating thickness can be varied just by varying the type ofcoating liquid. As seen from the results of tests 3, 6 and 7 which wereconducted under the same conditions except one factor, coating speed,the coating thickness can be varied by varying the coating speed; thefaster the coating speed becomes, the larger the coating thicknessbecomes. As seen from the results of tests 3, 8, 9 and 10 which wereconducted under the same conditions except only one factor, tensileforce, the coating thickness can be varied by varying the tensile force;the larger the tensile force becomes, the smaller the coating thicknessbecomes. As seen from the results of tests 10 and 11 which wereconducted under the same conditions except only one factor, flowresistance, the coating thickness can be varied by varying the flowresistance; the larger the flow resistance becomes, the larger thecoating thickness becomes.

[0113] Thus, the final coating thickness can be varied by controllingthe type (viscosity, solid content concentration) of coating liquid, thecoating speed, the pressing force applied to coating liquid atscraping-off portion (tensile force of web) and the flow resistance ofrecovering line (amount of valve opening).

[0114] When reusing the coating liquid having been recovered at thescraping-off portion through the pre-coating device without doing anyone of the coating liquid adjusting treatment and the filteringtreatment, the coating thickness obtained was the same as that of test 1and no dust was included in the coating liquid having been recovered.

[0115] These results show that the coating method and apparatus of thepresent invention enables the coating liquid having been scraped-off tobe reused without doing any one of the coating liquid adjustingtreatment and the filtering treatment while maintaining the features ofthe scraping-off type of coater, which is suitable to obtain a uniformand extremely thin coating film layer.

[0116] While the present invention has been described taking an examplein which the coating apparatus shown in FIG. 1 is used, the same resultscan be obtained when using the coating apparatus in which an integralcoating head and a back-up roller are combined, as shown in FIG. 4, orusing the coating apparatus of which scraping-off portion includes arotating rod, as shown in FIG. 5, or using a coating apparatus in whichseparately formed pre-coating portion and scraping-off portion and aback-up roller are combined in FIG. 6.

Example 2

[0117] As one of the suitable conditions when using the coatingapparatus of the present invention shown in FIG. 7, a suitable wetthickness t₁ of the coating liquid, which has been discharge too excessthrough the coating slit and applied to the web, was sought in relationto the surface condition of the coating film formed on the web.

[0118] The web, coating head, coating conditions used for the tests wereas follows. The composition of the coating liquid used is shown in Table4. TABLE 4 Composition of Coating liquid Part by weight Co substitutedBaFe 300 Average particle diameter 0.1 μm, Plates ratio 3.3, Averagethickness 0.03 μm, Anti-magnetic force 660 Oe Polyvinylchloride-acetate - Maleic anhydride copolymer 45 (polymerization degree450) Amyl stearate 10 Lecithin 3 Chrome oxide 5 Methyl ethyl ketone(MEK) 300 Cyclohexanone 300

[0119] As a web, PET film with 32 μm thick was used.

[0120] As a coating head, the one including two slits: a coating slitwith a slit gap of 0.2 mm and a recovering slit with a slit gap of 0.5mm was used. Of the lip surface at the tip of the coating head, the backlip surface was flat, and as for the doctor lip surface and therecovering lip surface, the doctor lip surface with the radius R of thecurvature of 10 mm or 4 mm and the recovering lip surface with theradius R of the curvature of 3 mm or 6 mm were properly combined andused.

[0121] As for the coating conditions, the coating liquid was applied tothe web at a coating speed (a running speed of the web) of 100 m/min toprovide a coating width of 0.3 m. The amount of the coating liquidsupplied Q₁ (mL/min) and the amount of the coating liquid sucked-upthrough the recovering slit Q₂ (mL/min) were varied among test 1 to test10 as shown in FIG. 2, and the difference Q=Q₁-Q₂ was applied to theweb. The values t₁, t₂, t shown in Table 5 were obtained by convertingthe values Q₁, Q₂ and Q into wet thickness. They are calculated by thefollowing equation, for example,

t ₁(mL/m ²)=Q ₁ (mL/min)/coating speed (m/min)/coating width (m).

[0122] The test results are shown in Table 5. TABLE 5 Q₁ Q₂ Q t₁ t₂ tSurface Condition of Test mL/min mL/min mL/min mL/m² mL/m² mL/m² CoatingFilm 1 250 200 50 8.3 6.7 1.7 Satisfactory 2 200 150 50 6.7 5.0 1.7Satisfactory 3 150 100 50 5.0 3.3 1.7 Satisfactory 4 100 50 50 5.3 1.71.7 Satisfactory 5 100 70 30 3.3 2.3 1.0 Satisfactory 6 90 60 30 3.0 2.01.0 Satisfactory 7 80 50 30 2.7 1.7 1.0 Streak defective 8 80 30 50 2.71.0 1.7 Slightly streak defective 9 60 20 40 2.0 0.7 1.3 Streakdefective 10 60 10 50 2.0 0.3 1.7 Streak defective

[0123] As seen from Table 5, if the wet thickness t₁ of the coatingliquid, which has been discharge too excess through the coating slit andapplied to the web, is 3 (mL/m2) or more, on the web from which theexcess amount of coating liquid has been sucked-up, a satisfactorycoating film surface, free from streak defect, is formed, as shown bythe results of tests 1 to test 6. However, when the wet thickness t₁ is2.7 (mL/m²) or less, as shown by the results of tests 7 to 10, streakdefect was observed on the coating film surface. Accordingly, the wetthickness t₁, as one of the suitable conditions when using the coatingapparatus of the present invention, is preferably 3 (mL/m²) or more.

[0124] As seen from the comparison between test 7 and test 8, if the wetthickness t₁ is the same, streak defect is less likely to occur when thethickness of the final coating film is larger.

[0125] According to the coating apparatus and method of the presentinvention, the coating liquid having been scraped-off can be reusedwithout doing any one of the coating liquid adjusting treatment and thefiltering treatment, while maintaining the features of the scraping-offtype of coater, which is suitable for obtaining a uniform and extremelythin coating film.

[0126] With the suction type coating apparatus of the present invention,a uniform and extremely thin coating film layer equal to or superior tothat formed by the scraping-off type of coating apparatus, which employsa doctor blade, can be formed. In addition, the coating liquid recoveredcan be reused without doing any one of the coating liquid adjustingtreatment and the filtering treatment.

[0127] Accordingly, the coating method and apparatus of the presentinvention is suitable to obtain a uniform and extremely thin magneticlayer which is required in manufacturing magnetic recording media, andparticularly suitable in extremely thin coating where the wet thicknessof the magnetic coating liquid applied to a web is 2 μm or less.

[0128] It should be understood, however, that there is no intention tolimit the invention to the specific forms disclosed, but on thecontrary, the invention is to cover all modifications, alternateconstructions and equivalents falling within the spirit and scope of theinvention as expressed in the appended claims.

What is claimed is:
 1. A coating method, comprising the steps of:pre-coating one side of a continuously running sheet substrate stripwith a coating liquid, using a pre-coating device, in an amount inexcess of that capable of obtaining an intended final coating thickness;and then scraping-off an excess amount of coating liquid with ascraping-off device provided on a downstream side along a runningdirection of the substrate relative to the pre-coating device, whereinthe coating liquid having been applied to the substrate is not opened tothe atmosphere during a time from the pre-coating until thescraping-off.
 2. The coating method according to claim 1, wherein thecoating liquid having been scraped-off is recovered through a recoveringline as a closed system and the recovered coating liquid is reused. 3.The coating method according to claim 2, wherein a desired coatingthickness is obtained by adjusting a fluid resistance of the recoveringline.
 4. The coating method according to claim 1, wherein a desiredcoating thickness is obtained by adjusting at least one of the amount ofthe coating liquid to be used for the pre-coating and an amount ofcoating liquid to be scraped-off.
 5. The coating method according toclaim 4, wherein the amount of the coating liquid used for thepre-coating and the amount of the coating liquid scraped-off aremeasured with flowmeters, respectively, and at least one of the amountsis adjusted to allow a difference between the two measured amounts toprovide the desired coating thickness.
 6. The coating method accordingto claim 1, wherein the desired coating thickness is obtained byadjusting a running speed of the substrate.
 7. The coating methodaccording to claim 1, wherein the desired coating thickness is obtainedby adjusting a viscosity of the coating liquid.
 8. The coating methodaccording to claim 1, wherein the desired coating thickness is obtainedby adjusting a solid content concentration of the coating liquid.
 9. Thecoating method according to claim 1, wherein the scraping-off device isconstructed so that the scraping-off device scrapes-off the excessamount of coating liquid by allowing a scraping-off portion thereof andthe substrate to be pushed relative to each other, thereby pressing thecoating liquid which has been applied to the substrate, the desiredcoating thickness being obtained by varying a pressing force applied tothe coating liquid.
 10. The coating method according to claim 9, whereinthe pressing force is obtained by lapping the substrate over thescraping-off portion and varied by adjusting a tensile force of thesubstrate exerted in the running direction of the substrate.
 11. Thecoating method according to claim 9, wherein the pressing force isobtained by arranging a back-up roller on the back side of thesubstrate, to which no coating liquid is applied, in such a manner as toface the scraping-off portion and is adjusted by adjusting the clearancebetween the scraping-off portion and the back-up roller.
 12. The coatingmethod according to claim 9, wherein the pressing force is obtained byarranging a back-up roller made of an elastic material on the back sideof the substrate, to which no coating liquid is applied, in such amanner as to face the scraping-off portion and is adjusted by adjustingat least one of a longitudinal elasticity modulus and a hardness of theback-up roller.
 13. The coating method according to claim 9, wherein thepressing force is obtained by arranging a spraying device which spraysfluid on the back side of the substrate, to which no coating liquid isapplied, in such a manner as to face the scraping-off portion and isadjusted by adjusting a spraying pressure of the spraying device. 14.The coating method according to claim 1, wherein the coating liquid is amagnetic coating liquid with magnetic particles dispersed therein.
 15. Acoating apparatus, comprising: a pre-coating device which pre-coats oneside of a continuously running sheet substrate strip with a coatingliquid in an amount in excess of that capable of obtaining an intendedfinal coating thickness; a scraping-off device which scrapes-off anexcess amount of coating liquid, the scraping-off device being arrangedon a downstream side along a running direction of the substrate relativeto the pre-coating device; and a closing mechanism which prevents thecoating liquid having been applied to the substrate from being opened tothe atmosphere between the pre-coating device and the scraping-offdevice.
 16. The coating apparatus according to claim 15, wherein: apre-coating portion of the pre-coating device and a scraping-off portionof the scraping-off device are integrated in a coating head; the closingmechanism is formed by allowing a lip surface of the coating head andthe substrate to be adjacent to each other; and the pre-coating portionis arranged on the upstream side of the coating head, and thescraping-off portion is arranged on the downstream side of the coatinghead.
 17. The coating apparatus according to claim 16, wherein: thepre-coating portion is provided with a coating slit for discharging thecoating liquid; and the scraping-off portion is provided with arecovering slit for recovering the coating liquid having beenscraped-off.
 18. The coating apparatus according to claim 16, furthercomprising: a pair of guide rollers which laps the substrate over thelip surface of the coating head, the pair of guide rollers beingarranged on upstream and downstream sides relative to the coating head,wherein the desired thickness is obtained by adjusting a tensile forceof the substrate exerted in the running direction of the substrate topress the coating liquid having been applied to the substrate.
 19. Thecoating apparatus according to claim 16, further comprising: a back-uproller which is arranged on a back side of the substrate, to which nocoating liquid is applied, in such a manner as to face the lip surfaceof the coating head, wherein the desired thickness is obtained byadjusting a clearance between the lip surface and the back-up roller topress the coating liquid having been applied to the substrate.
 20. Thecoating apparatus according to claim 17, wherein: the coating slit is incommunication with a supplying line for supplying the coating liquid tobe used for pre-coating; the recovering slit is in communication with arecovering line for recovering the coating liquid having beenscraped-off; the supplying line and the recovering line are providedwith flowmeters to measure the amount of the coating liquid used forpre-coating and the amount of the coating liquid having beenscraped-off, respectively; and a flow control device is provided whichcontrols at least one of a flow of the supplying line and a flow of therecovering line to allow a difference between the measured flows toprovide the desired coating thickness.
 21. The coating apparatusaccording to claim 17, wherein at least one of a doctor lip surfacebetween the coating slit and the recovering slit and a recovering lipsurface for scraping-off the excess amount of coating liquid, bothconstituting the lip surface of the coating head, has a shape of one ofa circular arc projecting toward the substrate, a combined surface of atleast two planes successively provided and forming a vertex projectingtoward the substrate, and a combined surface of a circular arc curvatureprojecting toward the substrate and a plane.
 22. The coating apparatusaccording to claim 21, wherein a relation R₂>R₃ holds, where R₂represents a radius of curvature of the doctor lip surface and R₃represents a radius of curvature of the recovering lip surface.
 23. Thecoating apparatus according to claim 16, wherein the scraping-offportion is provided with a rotating rod which comes in contact with thesurface of the substrate to which the coating liquid has been appliedand scrapes-off the excess amount of coating liquid.
 24. The coatingapparatus according to claim 15, wherein: a pre-coating portion of thepre-coating device and a scraping-off portion of the scraping-off deviceare separately formed; and the closing mechanism comprises a coveringmember which covers at least the surface of the substrate to which thecoating liquid has been applied, the covering member being arrangedalong a running route of the substrate between the pre-coating portionand the scraping-off portion.
 25. The coating apparatus according toclaim 24, wherein: the pre-coating portion is provided with a coatingslit for discharging the coating liquid; the scraping-off portion isprovided with a recovering slit for recovering the coating liquid havingbeen scraped-off; and a back-up roller is provided in such a manner asto face both of the pre-coating portion and the scraping-off portion.26. A coating method, comprising the steps of: pre-coating one side of acontinuously running sheet substrate strip with a coating liquid, usinga pre-coating device, in an amount in excess of that capable ofobtaining an intended final coating thickness; and then sucking-up anexcess amount of coating liquid using a suction device provided on adownstream side along a running direction of the substrate relative tothe pre-coating device, wherein the coating liquid having been appliedto the substrate is not opened to the atmosphere during a time from thepre-coating until the sucking-up.
 27. The coating method according toclaim 26, wherein a sucking force of the suction device is generatedwith a pump.
 28. The coating method according to claim 26, wherein thecoating liquid having been sucked-up with the suction device isrecovered through a recovering line as a closed system and the recoveredcoating liquid is reused.
 29. The coating method according to claim 26,wherein a wet thickness of the coating liquid applied to the substratethrough the pre-coating device is 3 mL/m² or more.
 30. The coatingmethod according to claim 26, wherein the desired coating thickness isobtained by adjusting at least one of the amount of the coating liquidto be used for pre-coating and an amount of the coating liquid to besucked-up.
 31. The coating method according to claim 30, wherein theamount of the coating liquid used for pre-coating and the amount of thecoating liquid sucked-up are measured with flowmeters, respectively, andat least one of the amounts is adjusted to allow a difference betweenthe two measured amounts to provide the desired coating thickness.
 32. Acoating apparatus, comprising: a pre-coating device which pre-coats oneside of a continuously running sheet substrate strip with a coatingliquid in an amount in excess of that capable of obtaining an intendedfinal coating thickness; a suction device which sucks-up an excessamount of coating liquid, the suction device being arranged on adownstream side along a running direction of the substrate relative tothe pre-coating device; and a closing mechanism which prevents thecoating liquid having been applied to the substrate from being opened tothe atmosphere between the pre-coating device and the suction device.33. The coating apparatus according to claim 32, wherein the suckingforce of the suction device is generated with a metering pump.
 34. Thecoating apparatus according to claim 32, wherein: a pre-coating portionof the pre-coating device and a suction portion of the suction deviceare integrated in a coating head; the closing mechanism is formed byallowing a lip surface of the coating head and the substrate to beadjacent to each other; and the pre-coating portion is arranged on theupstream side of the coating head, and the suction portion is arrangedon the downstream side of the coating head.
 35. The coating apparatusaccording to claim 34, wherein: the pre-coating portion is provided witha coating slit for discharging the coating liquid; and the suctionportion is provided with a recovering slit for recovering the coatingliquid having been sucked-up.
 36. The coating apparatus according toclaim 34, further comprising a pair of guide rollers which laps thesubstrate over the lip surface of the coating head, the pair of guiderollers being arranged on upstream and downstream sides relative to thecoating head.
 37. The coating apparatus according to claim 34, furthercomprising a back-up roller which makes the substrate adjacent to thelip surface of the coating head, the back-up roller being arranged on aback side of the substrate, to which no coating liquid is applied. 38.The coating apparatus according to claim 35, wherein: the coating slitis in communication with a supplying line for supplying the coatingliquid to be used for pre-coating; the recovering slit is incommunication with a recovering line for recovering the coating liquidhaving been sucked-up; the supplying line and the recovering line areprovided with flowmeters to measure the amount of the coating liquidused for pre-coating and the amount of the coating liquid having beensucked-up, respectively; and a flow control device is provided whichcontrols at least one of a flow of the supplying line and a flow of therecovering line to allow a difference between the measured flows toprovide the desired coating thickness.
 39. The coating apparatusaccording to claim 32, wherein: a pre-coating portion of the pre-coatingdevice and a suction portion of the suction device are separatelyformed; and the closing mechanism comprises a covering member whichcovers at least the surface of the substrate to which the coating liquidhas been applied, the covering member being arranged along a runningroute of the substrate between the pre-coating portion and the suctionportion.
 40. The coating apparatus according to claim 39, wherein: thepre-coating portion is provided with a coating slit for discharging thecoating liquid; the suction portion is provided with a recovering slitfor recovering the coating liquid having been sucked-off; and a back-uproller is provided in such a manner as to face both of the pre-coatingportion and the suction portion.